The present invention relates to a comparator circuit and a power supply circuit using the comparator circuit.
An electronic instrument such as a portable telephone which includes a liquid crystal display (LCD) panel operates using a removable battery as the power supply, for example. If an electric charge remains in a boosting capacitor when the battery is removed, an abnormal display may occur momentarily (display occurs momentarily; momentary operation). Therefore, a display-off operation of controlling the voltage applied to the liquid crystal according to a predetermined sequence is performed when removing the power supply from the electronic instrument.
The display-off operation must be performed when the user or the like suddenly removes the battery when the electronic instrument operates normally in the power-on state. In this case, the electronic instrument monitors the power supply voltage level, and performs the display-off operation when triggered by the monitoring result of the power supply voltage level when the electronic instrument judges that the battery has been removed.
The accuracy of the threshold value for judging that the battery has been removed is important when monitoring the power supply voltage level. If the threshold value is high and is close to the power supply voltage during power-on, the display-off operation starts due to noise or the like. If the threshold value is too low, the timing of judging that the battery has been removed is delayed, whereby the display-off operation cannot be completed.
As described above, when monitoring the power supply voltage level, it is preferable that the accuracy of the threshold value for judging that the battery has been removed be high.